The thermoplastic resins exhibit excellent processing characteristics and high shape degree-of-freedom. Accordingly, the thermoplastic resins are widely used for common industrial usages such as automobiles, electrical appliances, medical-care-related/bio-related appliances, and the like. Actually, the thermoplastic resins have prevailed up to such an extent that it can be said that there exists no field where the thermoplastic resins are not used. Namely, the thermoplastic resins have already become everyday-used familiar materials. At the beginning, the thermoplastic resins had been utilized as the alternatives for natural raw materials such as lumbers, papers, and the like. Nowadays, however, special products that cannot be produced without using the plastic materials are developed in large numbers. In view of this situation, if an optimum material is made full use of for the design and development using an optimum processing method, there exists a possibility that a never-before-possible new product will be able to be created and produced.
In particular, in accompaniment with the trend of product's structural complication and product's low-cost implementation in recent years, the design is performed where merits of the thermoplastic resins are made full use of. Also, the secondary-processing technologies are becoming important similarly. Of these secondary-processing technologies, attention has been focused on the laser welding by the prevalence of semiconductor lasers. The laser welding is a very clean joining technology that results in only a small extent of occurrence of the resin's burr. Accordingly, the research is also underway vigorously with medical-care-related/bio-related parts employed as the research target. Moreover, the features of the laser welding are as follows: Namely, the accuracy is high, and the distortion is small. This feature is made possible, because only the periphery of a target area, i.e., the laser-light irradiation area, is heated in a concentrated and rapid manner. Also, the welding/joining of complicated three-dimensionally-shaped objects or large-sized products is made possible by the scanning performed with laser light. Also, the product's low-cost implementation can be accomplished by a reduction in the number of the processing steps or the number of the parts. In general, however, a transparent resin and a black resin are used in the laser welding. As a result, the laser welding of mutually transparent resins is difficult to accomplish.
As the laser welding of mutually transparent resins, in PATENT LITERATURE 1, the explanation has been given concerning the following technique: Namely, a very thin transparent film, which exhibits a laser-light-absorbing absorption property, and which contains similar constituents to those of mutually transparent resin members, is caused to exist between these mutually transparent resin members. In this state, the laser irradiation is performed. This laser-irradiation operation makes it possible to weld even the mutually transparent resin members to each other. In PATENT LITERATURE 2, the explanation has been given regarding the following technique: Namely, toner or paint is caused to adhere onto one side of each of the mutually transparent resin members in accordance with a predetermined pattern. In this state, the laser irradiation is performed. This laser-irradiation operation makes it possible to weld even the mutually transparent resin members to each other. In PATENT LITERATURE 3, the disclosure has been made concerning the following technique: Namely, a concave/convex-portions surface is formed by roughing one side of the joining portion between the mutually transparent resin members. In this state, the laser irradiation is applied to this concave/convex-portions surface. This laser-irradiation operation allows even the mutually transparent resin members to be welded to each other.